Reference circuit enhancement for passive RFID tags

Abstract

A passive RF tag is disclosed in which the tag contains a tag oscillator for determining the modulation frequency of modulating the backscatter signal, and the tag oscillator frequency is stabilized when the tag modulates the backscatter signal.

1. A radio frequency (RF) transponder, comprising:
 an antenna; 
 a modulation circuit operatively coupled to the antenna and including an oscillator and a controlled current source, the oscillator being responsive to the controlled current source in determining a tag modulation frequency used to modulate a load on the antenna; and 
 a capacitor operatively coupled to the modulation circuit, wherein the capacitor is sufficiently large to stabilize the controlled current source against fluctuations that occur when the modulation circuit modulates the antenna. 
 
   
   
     2. The RF transponder of  claim 1 , wherein the controlled current source comprises a current mirror circuit providing a mirror voltage V m . 
   
   
     3. The RF transponder of  claim 2 , wherein the capacitor is operatively coupled to a line carrying the mirror voltage V m , and wherein the capacitor is sufficiently large that charges induced on the mirror voltage line change V m  by less than 5% when the modulation circuit backscatter modulates the antenna. 
   
   
     4. The RF transponder of  claim 3 , where the capacitor is sufficiently large that charges induced on the mirror voltage line change V m  by less than 2% when the modulation circuit backscatter modulates the antenna. 
   
   
     5. The RF transponder of  claim 2 , wherein capacitance C of the capacitor is greater than 2i/f 1 V m , wherein i is a characteristic current of the current mirror circuit and f 1  is the tag modulation frequency. 
   
   
     6. The RF transponder of  claim 5 , wherein capacitance C is greater than 3i /f 1 V m . 
   
   
     7. The RF transponder of  claim 5 , wherein capacitance C is greater than 5i/f 1 V m . 
   
   
     8. The RF transponder of  claim 2 , wherein capacitance C of the capacitor is selected such that the mirror voltage V m  charges to a steady state level in a period of time comparable to a single pulse of modulated power received by the antenna. 
   
   
     9. The RF transponder of  claim 2 , wherein the mirror voltage V m  further comprises a positive mirror voltage and a negative mirror voltage. 
   
   
     10. The RF transponder of  claim 9 , further comprising a rectification circuit operatively coupled to the antenna and providing a tag supply voltage, wherein the capacitor is coupled between the positive mirror voltage and the tag supply voltage. 
   
   
     11. The RF transponder of  claim 9 , wherein the capacitor is coupled between the negative mirror voltage and ground. 
   
   
     12. The RF transponder of  claim 1 , wherein the oscillator provides an oscillation frequency that is a multiple of the modulation frequency. 
   
   
     13. The RF transponder of  claim 1 , wherein the modulation circuit comprises a semiconductor chip and the capacitor comprises an external capacitor coupled to the semiconductor chip.